1. Field of the Invention
The present invention relates to a driving apparatus for light emitting diodes (LEDs), and more particularly to a pulse width modulation (PWM) driving apparatus for an LED.
2. Description of the Prior Art
FIG. 4 illustrates a conventional LED driving apparatus. The LED driving apparatus includes a power supply 10, a current limiting resistor 20 and an LED array 30 connected in series. The LED array 30 comprises a plurality of LEDs 31 arranged in an n×m matrix. In use, a forward current passing through each LED 31 determines its light output; therefore the light output of each LED 31 can be controlled by adjusting the forward current passing therethrough.
Referring to FIG. 5, a typical volt-ampere characteristic of a single LED 31 can be represented by the following equation:VF=Von+RsIf+(ΔV/ΔT)(T−25° C.)  (1),In this equation, VF and If represent a voltage applied to the LED 31 and a current flowing through the LED 31 respectively. Von is a threshold voltage of the LED 31, Rs represents equivalent resistance of the LED 31, (ΔV/ΔT) represents a temperature coefficient, and T represents junction temperature. A typical value of the temperature coefficient is 2V/° C. At a certain temperature, equation (1) can be simplified as:VF=Von+Rs If  (2), andtherefore a volt-ampere characteristic of the LED driving apparatus can be written as:U−m Von=I[R+(m/n) Rs]  (3), in whichU is a voltage of the power supply 10, and I is a driving current for the LED array 30. Equation (3) can be simplified as:U−Vx=I(R+Rx) . . . (4), and Vx=m Von, Rx=(m/n) Rs  (4)Equation (4) shows that the driving current I of the LED array 30 is not proportional to the voltage U of the power supply 10 due to the existence of the threshold voltage Von of each LED 31. That is, the driving current I of the LED array 30 does not change linearly when the voltage U of the power supply 10 changes linearly. Therefore it is difficult to precisely control the light output of the LED array 30.
Furthermore, when the number of LEDs 31 in the LED array 30 changes, both Vx and Rx are changed. Accordingly, either the voltage U of the power supply 10 or the resistance R of the current limiting resistor 20 must be changed in order to adjust the driving current I of the LED array 30. As described above, it is difficult to precisely control the driving current I (and thus the light output) by adjusting the voltage of the power supply, due to the nonlinear relationship between the driving current I of the LED array 30 and the voltage U of the power supply.